It is a modern trend of the vehicles to employ a power transmission device for vehicles which automatically executes the shifting operation or the clutch operation for easy driving. One of such power transmission devices for vehicles is a power transmission device used for diesel engine-mounted vehicles, employing a transmission of the type of parallel shaft gear mechanism operated by an actuator, arranging an automatic clutch, and interposing a fluid coupling between the engine and the automatic clutch. Upon interposing the fluid coupling for a diesel engine which produces a large torque in a region of small engine rotational speeds, in particular, it is made possible to start the vehicle by utilizing the slipping between the pump and the turbine in the fluid coupling. Namely, no delicate clutch operation is necessary that is required by the manual shift vehicles at the time of start, yet smooth start is easily accomplished absorbing fluctuation in the engine torque during idling and reducing vibration and noise.
The power transmission device interposing the fluid coupling will now be described with reference to a schematic diagram of FIG. 1. A fluid coupling 2 is joined to the rear of a diesel engine 1, and a transmission 4 having a parallel shaft gear mechanism is coupled thereto via a clutch 3. An output shaft of the transmission 4 is coupled to a propeller shaft that drives the wheels of a vehicle and is coupled to the wheels via a final reduction gear.
In the fluid coupling 2, there arranged a pump 21 integral with the output shaft of the diesel engine 1 and a turbine 22 integral with an input shaft 32 of the clutch 3. The two are basically coupled together by a lockup clutch 23 except at the start of the vehicle and, therefore, the output shaft of the diesel engine 1 is directly coupled to the input shaft 32 of the clutch 3. Further, the transmission 4 is a transmission of the type of ordinary parallel shaft gear in which a gear spline formed integrally with a gear is brought into mesh with a shift sleeve, and is equipped with a known synchronizing mechanism comprising a synchronizer ring or the like. The transmission 4 effects the gearshift by using a shift actuator 61 responsive to a shift instruction from an electronic control device. A vehicle which is not equipped with the electronic control device effects the gearshift by using a shift lever operated by a driver.
Referring to FIG. 2, the power transmission device of FIG. 1 employs a wet multiple disk clutch 3 as an automatic clutch. In the wet multiple disk clutch 3, there are alternately arranged a plurality of friction disks 34 attached to a member that is fitted by spline to the clutch input shaft 32 and a plurality of friction disks 35 attached to a member that is fitted by spline to the clutch output shaft 33. There is, further, arranged a clutch piston 36 for pushing them. If a working fluid is fed into a hydraulic chamber at the back of the clutch piston 36, the friction disks are pressure-contacted and the clutch engages. If the working fluid is drained, the clutch piston 36 is pushed by a spring 37 whereby the friction disks separate and the clutch is disengaged. A trochoidal oil pump 5 is provided between the fluid coupling 2 and the wet multiple disk clutch 3, and the working fluid is pressure-fed from the oil pump 5 into the interior of the wet multiple disk clutch 3 via passages formed in the circumference of the shaft of the turbine 22,and to the back of the clutch piston 36, too, through a control valve. The working fluid is pressure-fed from the oil pump 5 into the interior of the fluid coupling 2, too, via a fluid passage formed in the input shaft of the clutch.
FIG. 3 illustrates a circuit for feeding the working fluid. The working fluid is pressure-fed from the oil pump 5 to the fluid coupling 2 as well as to the wet multiple disk clutch 3. The pressure-fed working fluid is introduced into the interiors of the fluid coupling 2 and the wet multiple disk clutch 3 and is, further, used for controlling the lockup clutch 23 and the clutch piston 36. The fluid returning from these devices is cooled through an oil cooler 7 provided integrally with the radiator of the engine, and is refluxed into an oil tank.
The wet multiple disk clutch 3 is provided with a clutch control unit 31 (FIG. 1) which controls the amount of engaging the clutch in cooperation with an engine control unit 11 at the time of shifting the gear of the transmission 4. Here, at the time of shifting, the engine control unit 11 executes a control independently of the amount the accelerator pedal 62 is depressed.
The amount the clutch 3 is engaged is controlled by adjusting an electromagnetic valve 38 (FIG. 3) depending upon a duty ratio D of a pulse output by the clutch control unit 31 to thereby control the hydraulic pressure acting on the clutch piston 36 that pushes the friction disks. The clutch has been so set as to be engaged with a duty ratio D of 0% and disengaged with D of 100% in a steady state.
The clutch control unit 31 receives rotational speed signals transmitted from a rotational speed sensor 51 that detects the rotational speed of the input shaft 32 of the clutch 3 (rotational speed of the turbine 22 in the fluid coupling 2), rotational speed signals transmitted from a rotational speed sensor 52 that detects the rotational speed of the output shaft 33 of the clutch 3 (rotational speed of the input shaft of the transmission 4) and rotational speed signals transmitted from a rotational speed sensor 53 that detects the rotational speed of the output shaft of the transmission 4. The rotational speed signals are used for controlling the amount of engagement.
At the time of gearshift, the clutch control unit 31 gradually varies the amount of engaging the clutch 3 to avoid shift shock or engine stall caused by a sudden transmission of torque. For example, to engage the clutch 3 after the gears have been engaged, the clutch control unit 31 so controls the duty ratio as to gradually increase the amount of engaging the clutch 3. The clutch 3 undergoes the slipping in a state of a so-called half-engage clutch so that the engine rotational speed and the input shaft rotational speed of the transmission 4 are gradually brought into agreement with each other. When the clutch is completely engaged (D=0%), the slipping amount becomes zero, and the diesel engine 1 is placed in a state where it is directly coupled to the input shaft of the transmission 4.
Here, in a region where the amount the clutch 3 is engaged is not still reaching the half-engaged state of the clutch, the torque is not almost transmitted even if the hydraulic pressure is elevated from the state of being disengaged. To quickly accomplish the disengagement, therefore, the above region must be passed over within a short period of time. Further, the wet multiple disk clutches 3 really mounted on the individual vehicles have their own differences. Besides, even the same clutch undergoes a change due to aging. Therefore, the amount of engagement where the clutch starts half-engaging varies depending upon each clutch. Accordingly, a hydraulic pressure value at a point where the clutch starts half-engaging must be learned at regular intervals or, in other words, a duty ratio that serves as the amount of engagement which starts substantially transmitting the torque must be learned at regular intervals for each of the individual vehicles. By using a half-engage clutch learned value that is successively updated, the clutch control unit 31 makes it possible to execute the clutch control correctly and quickly.
The state where the clutch is half-engaged has generally been learned in the vehicles equipped with an automatic clutch. For example, there has been known a method of learning the amount of engagement with which the transmission input shaft starts rotating while gradually increasing the amount of engaging the clutch. Further, a method of learning the amount of engagement in a state where the clutch 3 is half-engaged in a power transmission device equipped with a fluid coupling has been disclosed, for example, in JP-A-2002-295529. According to this publication, the state where the clutch is half-engaged is learned when the vehicle is at a halt with the gears of the transmission 4 being engaged and the diesel engine 1 rotating. The wet multiple disk clutch 3 is disengaged, and the lockup clutch 23 of the fluid coupling 2 is disengaged, too.
Since the wet multiple disk clutch 3 has been disengaged, the turbine 22 of the fluid coupling 2 is rotating nearly at the same rotational speed as the diesel engine 1 being dragged by the pump 21 despite the vehicle is at a halt and the output shaft 33 of the wet multiple disk clutch 3 remains stationary. Referring to FIG. 7, the output duty ratio D of the clutch control unit 31 is decreased from this state and the amount of engaging the wet multiple disk clutch 3 is increased. Since the output shaft 33 of the wet multiple disk clutch 3 remains stationary, the rotational speed of the turbine 22 integral with the input shaft 32 of the wet multiple disk clutch 3 decreases with an increase in the amount of engagement, i.e., with an increase in the amount of transmitting the torque. A duty ratio of when the amount of decrease in the rotational speed of the turbine 22 has reached a predetermined value with respect to the rotational speed of the diesel engine 1 (rotational speed of the pump 21) is stored in the clutch control unit 31 as a half-engage clutch learned value duty ratio.